The Mage Plate gives you +1 MP, but -5% attack bonus, every time you level up. If you convert the item, both the benefit and penalty goes away. Basically it's the Attack->MP booster prep on steroids. You're not missing much; it's pretty much only viable for pure casters and still doesn't perform well as a prep since the Crystal Ball is its superior by a significant margin. Human hybrids with the HP->Attack booster prep can also make good use of it, but it's generally too much effort to get that combo off the ground.

I don't know how balanced it would be, but it would at least be fun to have Transmuters do more actual transmutation. Like, every wall destroyed turns into a gold pile a la the Transmutation Scroll - but with some (very) low % chance to get a big gold pile, an extra HP/MP/Atk bonus, or even a (non-Elite) shop item. Give them the ability to polymorph monsters. X number of bloodpools can be collected and turned into a potion. Items can be transmuted into other items (of equivalent value). One or more of those abilities could make for an interesting class.